Complexity in elucidating crustal thermal regime in geodynamically affected areas: A case study from the Deccan large igneous province (western India)

Abstract

Complexity in the earth’s crustal structure plays an important role in governing earth’s thermal and geodynamic behavior. In the present study, an attempt has been made taking insights from our recent geological, geochemical, petrophysical and geophysical findings from specially drilled deep boreholes, to understand the lithospheric thermal evolution of the highly complex western India, which forms the core region of the Deccan large igneous province. This region was severely affected by the Deccan volcanic eruptions 65 Ma ago, which resulted in a totally degenerated, reworked and exhumed mafic crust, which presently contains several Tertiary basins with proven hydrocarbon reserves. Our detailed case study from the disastrous 1993 Killari earthquake (Mw 6.3) region, apart from some other geotectonically important localities like seismically active 2001 Bhuj and 1967 Koyna earthquake regions together with Tertiary Cambay graben, indicate that the western part of India, is perhaps one of the warmest segments of the earth. It is characterized by an average high mantle heat flow and Moho temperatures of about 43 mW/m2 (range: 31-65 mW/m2) and 660°C (range: 540-860°C) respectively. Estimated thickness of the lithosphere beneath these areas varies from as low as about 45 km to 100 km. Consequently, melting conditions in certain segments are expected at extremely shallow depths due to asthenospheric swell, like northern part of Cambay basin and Bhuj seismic zone beneath which only about half of original crystalline crust now remains due to sub-crustal melting and massive exhumation of deeper crustal layers. Sustained thermal heating and rise of isotherms appear to have resulted in substantial enhancement of hydrocarbon generation and maturation processes in Tertiary sediments. The present study highlights the need of an integrated geological, geochemical and geophysical study, if reasonably accurate deep crustal thermal regime is to be investigated.